# Multi-time-zone meeting planner

> One instant, read in every attendee's IANA time zone: local time, UTC offset, day shift, and a working-hours flag. DST is whatever the zone database says for that date, which is exactly why you plan with a real one.

- Tool: https://ronutz.com/en/tools/timezone-meeting-planner
- Family: Encoding & data

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## What it does

Give it one meeting instant - with Z or an explicit offset, because a meeting time without one is ambiguous - and a list of IANA time zones, one per line. It reads that single instant in every zone: local date and time, weekday, the UTC offset tzdata reports for that date, the date shift relative to the first zone, and a working-hours flag. The zone data is your browser's own IANA database, the same one the operating system keeps current.

## The flags, stated

Working hours are flagged for 09:00-17:59 local, Monday to Friday - the conventional envelope, named rather than hidden. A date-shift column calls out attendees sitting on a different calendar date, the classic Tokyo-joins-tomorrow surprise worth saying out loud in the invitation.

## Why the date matters

Daylight saving is whatever the zone's rules say for that specific date: Berlin is UTC+2 in July and UTC+1 in January; Sao Paulo is UTC-3 year-round since Brazil abolished DST in 2019. Planning with a real date is the whole point.

## Standards and references

- [IANA — Time Zone Database](https://www.iana.org/time-zones) - the zone and DST rules the runtime carries
- [ECMA-402 — Intl.DateTimeFormat and time zone handling](https://tc39.es/ecma402/#datetimeformat-objects) - how the readings are computed
- [RFC 3339 — Date and Time on the Internet: Timestamps](https://www.rfc-editor.org/rfc/rfc3339) - the instant's input format

## Related reading

- [Time Arithmetic and Time Zones: Exact Durations, Honest Meetings](https://ronutz.com/en/learn/time-arithmetic-and-time-zones.md): Why 'plus one month' has no exact answer, what ISO 8601 durations actually promise, and how one UTC instant becomes four different wall clocks. The DST trap, the day-shift trap, and the case for planning meetings with a real date.